Building Blender - single lib folder for many different builds?
OK, I figured it out my self.

There are couple of lines in CMakeLists.txt that set the path to the lib folder one level relative to the source files folder (carried by the CMAKE_SOURCE_DIR variable).
To make it work with my example folder structure simply adding one more level solved the problem:

    if(NOT DEFINED LIBDIR)
        # Setup 64bit and 64bit windows systems
        if(CMAKE_CL_64)
            message(STATUS "64 bit compiler detected.")
            set(LIBDIR ${CMAKE_SOURCE_DIR}/../../lib/win64_vc12)
        else()
            message(STATUS "32 bit compiler detected.")
            set(LIBDIR ${CMAKE_SOURCE_DIR}/../../lib/windows_vc12)
        endif()
    else()
        message(STATUS using LIBDIR  ${LIBDIR})
    endif()


My real folder structure was a bit more complex and the common lib location was off the branch with sources so I made it a bit more complicated:

    # added this near the top of CMakeLists.txt for easy reference
    if(WIN32)
        set(COMMON_LIBPATH "C:/some_complex_path/CommonStuff/Blender/lib")
    endif()

    ...

    if(NOT DEFINED LIBDIR)
        # Setup 64bit and 64bit windows systems
        if(CMAKE_CL_64)
            message(STATUS "64 bit compiler detected.")
            if(COMMON_LIBPATH)
                message(STATUS "Using common lib path: " ${COMMON_LIBPATH})
                set(LIBDIR ${COMMON_LIBPATH}/../lib/win64_vc12)
            else()
                set(LIBDIR ${CMAKE_SOURCE_DIR}/../lib/win64_vc12)
            endif()
        else()
            message(STATUS "32 bit compiler detected.")
            if(COMMON_LIBPATH)
                message(STATUS "Using common lib path: " ${COMMON_LIBPATH})
                set(LIBDIR ${COMMON_LIBPATH}/../lib/windows_vc12)
            else()
                set(LIBDIR ${CMAKE_SOURCE_DIR}/../lib/windows_vc12)
            endif()
        endif()
    else()
        message(STATUS using LIBDIR  ${LIBDIR})
    endif()


You have to edit the CMakeLists.txt file in each source code folder (e.g. the standard "blender" folder).

As I am targeting Windows this applies to, you guessed it, Windows.